This invention relates to a method of preparing coal for safe storage and transport and simultaneously improving its fuel value. More particularly it relates to a method of heat-treating coal which involves passing it through a molten metal bath to produce a solid fuel that possesses a higher heating value and can be stored or transported without posing potentially serious combustion hazards and associated economic losses through excessive autogenous heating.
When exposed to the atmosphere, freshly mined low-rank coals--especially those classed as lignites and subbituminous coals--will rapidly chemisorb oxygen and consequently, even when held in compacted stockpiles through which relatively little air can circulate freely, tend to heat up until igniting. The same is true of chars prepared from such coals by conventional carbonization at temperatures below -700.degree. C. Commonly referred to as "spontaneous" or (more correctly) "autogenous" heating, this presents a serious hazard whenever substantial tonnages of low-rank coals are stockpiled or transported in open freight cars or in ships' holds.
The primary cause of autogenous heating lies in air-oxidation of the coal, which converts its organically-bound carbon into carbon dioxide with an ultimate heat release of 395 kJ per mole. If the coal contains a relatively high proportion (say, more than 1%), of inorganic sulphur, autogenous heating can also arise from oxidation of pyrite, which is believed to cause formation of a pyrophoric ferrous sulphate as an intermediate reaction product (cf. Mapstone, G. E.; 1954. Chem. Ind. (London), 658) and which ultimately liberates some 840 kJ per mole. Since oxidation rates will approximately double with each 10.degree. C. rise in temperature, these heat releases, if not quickly dissipated into the surrounding air, will promote a self-accelerating oxidation process and cause the coal to heat up progressively more quickly toward its ignition temperature.
However, at normal ambient temperatures, carbon oxidation rates are too small to initiate this sequence, and it is therefore also generally accepted that autogenous heating is in practice triggered by heat released when a dry or partly dried coal is wetted by water. Such "heats of wetting" will, in other words, raise the temperature of the coal to levels at which the carbon oxidation rates can eventually lead to ignition. This mechanism, first proposed by Berkowitz and Schein (1951) Fuel, 30, 94, on the basis of laboratory measurements of heats of wetting, has been confirmed by others (cf., for example, Hodges & Hinsley, 1963, Trans. Inst. Min. Engrs., 123, 211; Hodges & Acherjee, 1966, ibid., 126,121; Battacharyya et al, 1969, Min. Engng., 101, 274; Guney, 1971, Bull. Can. Inst. Min. Metall., 64, 138; and Shea & Hsu, 1972) and is independently supported by observations on bulk storage piles. For example, Erdtmann & Stoltzenberg (1908) Braunkohle, 7, 69; Threllfall (1909) and Hoskins (1928), Purdue Univ. Expt. Stn., Bull. No. 30, among many others, have reported that autogenous heating commonly begins (1) after rain following a period of dry sunny weather, (2) when dry coal is placed on wet ground, or (3) when wet coal is loaded onto an established, partially dried-out stockpile. In the latter cases, heating invariably begins at the interface between wet and dry material.
There does not yet exist a definitive laboratory test for determining whether a particular coal is likely to pose significant hazards through autogenous heating. However, since heats of wetting under otherwise similar conditions are proportional to the extent of the wetted surface (which is in turn proportional to the porosity of the coal), most authorities agree that the probability of autogenous heating is statistically related to the "capacity" moisture (or "bed" moisture) content of coal. This parameter is defined as the amount of water which a coal will hold when in equilibrium with air at 98% relative humidity--i.e. when fully water-saturated; it directly reflects the porosity of the coal; and, like the porosity, decreases rapidly with increasing rank (from over 30% among lignites to 18-22% among subbituminous coals and less than 10% among bituminous coals).
Like conclusions from laboratory observations on the effect of wetting of dry or partly dried coal on oxidation rates, this inference, too, is confirmed by practical experience. Hazards from autogenous heating decrease rapidly with increasing rank and only pose serious problems among lignites and subbituminous coals. High volatile bituminous and coals of higher rank are not, as a general rule, subject to significant autogenous heating; and it follows therefore that a lignite or subbituminous coal (with 35% and 20% bed moisture content, respectively) could be rendered relatively "safe" by procedures which, without causing chemical changes that enhance their reactivity toward oxygen, permanently reduce its capacity moisture content to, say, less than 10-12%.